High efficiency gas-fired combustion appliances usually have so-called fan-assisted combustion systems, wherein a fan or blower is used to move combustion reactants and products through the system. The blower energy can enhance performance of the appliance in several ways. It can enhance mixing of the fuel and air. It can enhance heat transfer by forcing the convective process. It can enhance disposal of the combustion products by moving them out of the appliance through a suitable vent.
A common approach to design of such systems is to use the blower to "pull" the flow through the system. Such an approach is often referred to as induced draft. One advantage of induced draft is that all parts of the system are at negative pressure relative to the atmosphere, and a leak will not result in heat or combustion products escaping into the environment. On the other hand, the blower must handle combustion gases which may be hot and moist.
Suitable components for high efficiency equipment have been produced commercially for several years. However, commonly available blowers have been designed for residential space heating units, which have energy input rates limited to about 150,000 Btu/h. Blowers for equipment of higher input rate are not readily available.
If the appliance is to also have especially good combustion quality, such as very low emission of oxides of nitrogen, it is advantageous to employ a "pre-mix" combustion system, i.e. one in which the air and fuel are mixed completely before reaching the flame zone. Bunsen-type burners, in which only a portion of the necessary air is provided before the flame zone, typically have higher emission of oxides of nitrogen than pre-mix burners.
Effective pre-mixing of fuel and air requires energy, and such energy is in addition to that required to meet the basic flow requirement of the combustion system.
An appliance having high energy input rate and a pre-mix burner would therefore require a combustion blower with considerably more flow and pressure capability than is commonly available for typical furnaces. Since the power requirement of such a blower is proportional to the product of flow and pressure, a blower providing three or four times the flow and two or three times the pressure might easily require five to ten times as much operating power.
In the design of a gas appliance, the flow through the combustion system cannot be reduced significantly. The reactant or fuel gas flow rate must provide the specified energy input rate. Air flow can be no lower than the stoichiometric requirement for oxygen, and in practical combustion systems has to be substantially higher. Therefore, the only way to significantly decrease combustion blower power requirement is to reduce the pressure requirement, requiring careful attention to all dissipative elements, such as heat exchangers, burners or mixing devices.
In very high efficiency appliances, combustion products are cooled below their dew point (about 130.degree. F. or about 54.degree. C., varying with excess air), and liquid water is condensed. This water is somewhat corrosive, and components contacted thereby, such as a combustion blower must be constructed of corrosion-resistant materials. Also, accumulated water must be collected and disposed of by means of suitable drains.
Manufacturers of combustion blowers have developed products providing high flow and pressure by applying high-speed motors to their existing blowers. Typically, a brushless d.c. motor, capable of operating at 5,000 RPM or more is applied to a blower originally intended for operation with 3,000 RPM AC motors.
Operation at such speeds increases the noise produced by the blower. An element of this noise may be the "blade passage" tone, comparable to the slapping noise of a helicopter blade or the whining of a jet engine. In the case of combustion blower systems, annoying tones of this kind are produced by the motion of the tips of the blower blades past a stationary object, such as the cutoff of the blower housing. One available blower has nine blade tips and operates at about 5000 RPM. This results in an audible tone of 45,000 cycles per minute or 750 Hz, which can be very annoying.